As is known, commonly used helicopter transmission lubricating pumps have a drive shaft fitted with a gear that meshes with a gear in the transmission. The most common arrangement is what is known as a “cartridge”, i.e. the pump is housed partly inside a cylindrical seat coaxial with the drive shaft and formed in a housing, which has two sections on opposite sides of the pump and connected to the lubricating circuit intake and delivery pipes respectively.
Since relatively little power is normally demanded of the pump, torque transmitted to the drive shaft is also relatively low, so the drive shaft can simply be supported by bushings, with no need for rolling bearings.
The seat in the housing must be wide enough to permit passage of the drive shaft and its gear, the size of which is inversely proportional to the required rotation speed of the drive shaft.
In helicopter transmissions, rotation speeds are relatively high, so volumetric efficiency is low. That is, the pumping chambers rarely manage to fill completely, because of the short length of time they remain connected to the intake section. Moreover, excessive rotation speed may cause cavitation phenomena, resulting in rapid wear and unreliability of the pump.